1. Field of the Invention
The present invention relates to an image heating apparatus that is applied to an image forming apparatus such as a copying machine, a laser printer and a facsimile machine.
2. Related Art
Conventionally, as a fixing apparatus, a heat roller system has been generally employed, in which a recording material is passed through a pressing-contact portion of a pair of rollers, one or both of which being provided with a heater, to thereby fix an image on the recording material. In recent years, a film heating system disclosed in Japanese Patent Applications Laid-open Nos. 63-313182, 4-44075 and 4-44077 has also been employed, in which a pattern of a heating element is formed on a ceramic base material to create a heater and the heater is caused to generate heat, thereby heating a body to be heated via a thin film.
Since a heating body of low heat capacity can be used as a heater in a fixing apparatus of such a film heating system, waiting time can be further reduced (printing can be started more quickly) compared with the conventional heat roller system. In addition, since printing can be started more quickly, preheating becomes unnecessary during non-print operation, whereby comprehensive saving of power can be realized.
Incidentally, a base material of a heater is manufactured by dividing a plate that is large enough for cutting out several pieces of base material therefrom. The plate is divided by using a diamond cutter, by laser cutting or by shearing the plate along holes formed as perforation. However, it is likely that burrs or harmful protrusions exist on an end surface in any case.
Inserting such a heater into a nip in which a heat resistant film is sliding, with its ends in pressed contact with the nip, involves a significant risk from the viewpoint of abrasion durability.
That is, if a nip width is to be expanded to exceed a width of the heater, it is necessary to cut the base material of the heater by a method in which burrs or harmful protrusions are not generated on the end surface or implement processing for removing the burrs or harmful protrusions after cutting the base material of the heater, or use a heat resistant film which is not susceptible to burrs or harmful protrusions even if such burrs or protrusions exist.
Thus, in a generally adopted configuration, the heater width is set smaller than the nip width in order to avoid the secondary processing or the technical risks described above.
On the other hand, with increasing interest in the environmental issues in recent years, further energy saving is being sought. Under such circumstances, since half or more of power required for the printer is consumed by the fixing apparatus even in an electrophotographic printer having a fixing apparatus of the film heating system, such large power consumption by the fixing apparatus is now regarded as a problem. Thus, further saving of power by the improvement in thermal efficiency is sought even in the fixing apparatus of the film heating system.
As effective means for improving thermal efficiency of a fixing apparatus, there may be conceived of increasing a pressurizing force to enlarge a nip width, to thereby obtain a larger pressurizing and heating area.
However, since the task of making the nip width larger than a heater width involves the technical difficulties mentioned above, the nip width can be increased only to the extent that it does not exceed the heater width.
Next, as another effective means for improving thermal efficiency of a fixing apparatus, there may be conceived of enlarging a width of a heating element in a heater to obtain a larger heating area.
However, it is needless to mention that heat is not transmitted to a recording sheet if the heating element sticks out from a nip area. Thus, it is necessary to enlarge the nip width as the heating element is enlarged. However, due to the above-mentioned reason, the nip width can be enlarged only to the extent that it does not exceed the width of the heater. Therefore, it is necessary to set a dimension such that a relationship that the nip width is within the heater width and the heating element width is within the nip width can be always maintained.
FIGS. 4A, 4B, and 4C show a fixing apparatus according to a related art of the present invention. FIG. 4A is a perspective view, FIG. 4B is a side view and FIG. 4C is a view showing a nip portion.
In FIGS. 4A to 4C, reference numeral 100 denotes a heat resistant film, 102 denotes a heater, 102a denotes a heating layer (H is a width of the heating layer), 102b denotes a glass layer, 113 denotes a heater holding member, 104 denotes a metal stay, 115 denotes a resin member, 115a denotes a regulating surface for regulating the heat resistant film 100 in a width direction thereof, 115b denotes a pressure application location, 106 denotes a pressurizing roller, and 117 denotes a pressurizing roller supporting member. A pressure is applied on the pressure application location 115b of the resin member 115 by not-shown pressurizing means, whereby the heater 102 can be brought into pressed contact with the pressurizing roller 106 via the metal stay 104 and the heater holding member 113. The pressurizing roller 106 is subjected to rotational driving force by not-shown rotational driving means to rotate in a direction shown in the figure, whereby the pressurizing roller 106 is capable of conveying a recording sheet.
As shown in FIG. 4B, in a generally adopted construction, the pressurizing roller 106 is fit and positioned in the pressurizing roller supporting member 117 through a bearing, and the heater 102 is positioned in the heater holding member 113, the heater holding member 113 is fit and positioned in the resin member 115 within a portion A in the figure and the resin member 115 is fit and positioned in the pressurizing roller supporting member 117 within a portion B in the figure.
Therefore, dimensional allowances X1, X2, X3 and X4 shown in FIG. 4C are set such that the above-described relationship that the nip width (N) is within the heater width and the heating element width is within the nip width can be always maintained, even if loose fitting or a problem of inconsistencies in manufacturing occurs in assembling the above-mentioned members.
That is, the dimensional allowances X1, X2, X3 and X4 take relatively large values, which results in increased size of the heater.
Here, if the width of the heater 102 is enlarged, it is possible to enlarge the nip width without including ends of the heater 102 in the nip. However, if the width of the heater and the size of the heater holding member are enlarged, the entire heating apparatus becomes larger and, at the same time, a heat capacity of the apparatus itself is increased, which actually spoils the effect of improved thermal efficiency. Thus, it is not desirable to enlarge the width of the heater and the size of the heater holding member.
The present invention has been devised in view of the above-mentioned drawbacks, and it is an object of the present invention to provide an image heating apparatus in which thermal efficiency is improved to realize saving of power.
Another object of the present invention is to provide an image heating apparatus including: a heater; a film sliding against said heater; a roller forming a nip together with the heater via the film; wherein a recording material bearing an image is nipped and conveyed in the nip and the image on the recording material is heated by heat from the heater via the film, a holding member for holding the heater; and a supporting member for supporting the roller, wherein the holding member is directly supported by the supporting member.
Other objects of the present invention will become apparent from the following description.